1. Field of the Invention
The present invention relates generally to a method of reproducing information on an optical recording medium, and more particularly, to a reduction of noise in a reproduced information signal and a reduction of crosstalk in multiplex recording/reproducing.
2. Description of the Background Art
In recent years, research and development have been made on an optical recording technique enabling high density recording. Such optical recording technique employs various disks such as a reproduction only compact disk (CD), a video disk, an erasable magnet-optical disk and an optical disk using functional dye. Light output from a semiconductor laser device used as a light source for reproducing information on a commercially available optical recording medium at present, in particular, a semiconductor laser device oscillating in a single mode, inevitably includes mode hop noise and back-talk noise (hereinafter referred to as light source noise). A reproduced signal therefore includes light source noise, so that a carrier/noise (C/N) ratio of the reproduced signal is reduced to prevent highly reliable reading of information.
In order to solve such problem, a semiconductor laser device oscillating in a single mode employs a method of superimposing a high frequency of about 1GHz on a current for driving the laser device to bring the laser device into a multi-mode oscillation state. The method, requiring an oscillation circuit for generating a high frequency to be provided in proximity to the semiconductor laser device, leads an increase in size of the semiconductor laser device and a pick-up unit including an objective lens. In addition, a shield for covering the pick-up unit should be provided in order to prevent leakage of a high frequency.
Consideration is given to the use of a short wavelength laser device utilizing secondary harmonics generation (SHG). This does not allow the above-described method of interposing a high frequency on the drive current to reduce noise.
Meanwhile, search has been carried out in recent years on the use of a photochromic material in a recording layer of an optical recording medium. The photochromic material undergoes a photochemical reaction in response to irradiation by light having a predetermined wavelength, whereby its molecular structure is changed and optical characteristics such as light absorbance are changed accordingly. The photochromic material also has a property of restoring its original molecule structure by irradiation of a light having another predetermined wavelength. Such property is used for reproduction of information recorded on an optical recording medium including a photochromic material, wherein the optical recording medium is irradiated with a laser beam for reproduction with low power, and an absorbance is changed in response to a photochemical reaction following a photochemical reaction at the time of recording is detected as a reflectance change.
Since no perfect surface smoothness can be obtained of a recording medium in a process of manufacturing the same (at the time of forming grooves, for example), such reproduction of information results in a reproduced signal including noise (hereinafter referred to as medium noise) due to surface non-smoothness of the optical medium, so that a reproduced signal of low noise (high C/N) cannot be obtained.
Japanese Patent Laying Open-No. 61-203450 discloses using the property of a photochromic material for multiplex recording (wavelength multiplex recording) of information on an optical recording medium including a plurality of photochromic materials having optical wavelength ranges for light absorption different from each other.
With reference to FIGS. 1A, 1B and 1C, there are shown light absorption characteristics of two kinds of photochromic materials (a) and (b) having their respective wavelength ranges for light absorption. In these diagrams, the abscissa represents a wavelength of light, while the ordinate represents a light absorbance. FIG. 1A shows the dependency of absorbance in the photochromic material (a) absorbance upon wavelength. The photochromic material (a) is changed by irradiation of a light having a predetermined wavelength to have the maximum absorbance at a wavelength of .lambda..sub.a and to have the maximum absorbance at .lambda..sub.a canceled by irradiation of a light having another predetermined wavelength. FIG. 1B shows the dependency of absorbance in the photochromic material (b) upon wavelength. The photochromic material (b) is similar to the material (a) but has a different maximum absorbance at wavelength .lambda..sub.b which is different from .lambda..sub.a. FIG. 1C shows absorption characteristics of an optical recording medium including both of the photochromic materials (a) and (b). Such optical recording medium allows two-wavelength multiplex recording based on existence/no existence of maximum absorbances at wavelength .lambda..sub.a and .lambda..sub. b.
However, since the photochromic material (b) generally absorbs a small amount of light even at wavelength .lambda..sub.a, when information recorded in the photochromic material (a) by using a light having wavelength .lambda..sub.a is reproduced according to a light absorption change, information recorded in the photochromic material (b) is mixed in a reproduced signal (generation of cross-talk). This is also the case with reproduction, by using a light having wavelength of .lambda..sub.b, of the information recorded in the photochromic material (b) by the light having wavelength .lambda..sub.b.
Further known is a method of polarization multiplex recording by utilizing other material properties of a photochromic material. According to this method, information is recorded by irradiating an optical recording medium including uniformly distributed photochromic materials with a linearly polarized light for recording. The recorded portion has a large absorbance change with respect to a light having the same wavelength and the same polarization plane as those of the light for recording. On the other hand, the recorded portion has a small absorbance change with respect to a light having the same wavelength as that of the light for recording and having a polarization plane vertical to the polarization plane of said light. Duplex recording therefore can be made on the same portion of an optical recording medium by using linearly polarized lights orthogonal to each other.
This method, however, also causes cross-talk (polarization multiplex cross-talk) similar to that in the above-described wavelength multiplex recording. This is because a photochemical reaction is more likely to occur when transition moment that photochromic molecules have are in parallel with a polarization plane of a light, while a slight photochemical reaction occurs even when they are not in parallel with each other.